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  • 1.
    Khoshniyat, Fahimeh
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Optimization-Based Methods for Revising Train Timetables with Focus on Robustness2016Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    With increase in the use of railway transport, ensuring robustness in railway timetables has never been this important. In a dense railway timetable even a small disturbance can propagate easily and affect trains' arrival and departure times. In a robust timetable small delays are absorbed and knock-on effects are prevented effectively. The aim of this thesis is to study how optimization tools can support the generation of robust railway traffic timetables. We address two Train Timetabling Problems (TTP) and for both problems we apply Mixed Integer Linear Programming (MILP) to solve them from network management perspectives. The first problem is how robustness in a given timetable can be assessed and ensured. To tackle this problem, a headway-based method is introduced. The proposed method is implemented in real timetables and evaluated from performance perspectives. Furthermore, the impact of the proposed method on capacity utilization, heterogeneity and the speed of trains, is monitored. Results show that the proposed method can improve robustness without imposing major changes in timetables. The second problem addressed in the thesis is how robustness can be assessed and maintained in a given timetable when allocating additional traffic and maintenance slots. Different insertion strategies are studied and their consequences on capacity utilization and on the properties of the timetables are analyzed. Two different insertion strategies are considered: i) simultaneous and ii) stepwise insertion. The results show that inserting the additional trains simultaneously usually results in generating more optimal solutions. However, solving this type of problem is computationally challenging. We also observed that the existing robustness metrics cannot capture the essential properties of having more robust timetables. Therefore we proposed measuring Channel Width, Channel Width Forward, Channel Width Behind and Track Switching.

    Furthermore, the experimental analysis of the applied MILP model shows that some cases are computationally hard to solve and there is a need to decrease the computation time. Hence several valid inequalities are developed and their effects on the computation time are analyzed.

    This thesis contains three papers which are appended. The results of this thesis are of special interests for railway traffic planners and it would support their working process. However, railway traffic operators and passengers also benefit from this study.

    List of papers
    1. Robustness Improvements in a Train Timetable with Travel Time Dependent Minimum Headways
    Open this publication in new window or tab >>Robustness Improvements in a Train Timetable with Travel Time Dependent Minimum Headways
    2015 (English)Conference paper, Published paper (Other academic)
    Abstract [en]

    In a railway network with dense traffic, trains’ scheduled arrival and departure times arehighly dependent on each other and even a small delay easily propagates to subsequenttrains using the same infrastructure resources. In the current paper a given timetable is comparedto a modified timetable, where the assigned minimum time slot in the traffic for aservice is linearly increasing with the service’s travel time. The underlying assumption isthat trains lose precision as they travel longer and catching a fixed-size time slot is easier atthe beginning of the journey. Real world observations confirm this assumption as well. Theaim of this study is to verify the improvement in the robustness of those timetables that aremodified with respect to the idea of travel time dependent reserved time slots for the arrivaltimes of trains and to compare the results with the initial timetables. Numerical experimentsare conducted on a selected double track segment of the Swedish Southern mainline. Fourtimetable case studies are considered for the experiments: off-peak hours and peak hoursin 2011 and 2014, respectively. Each timetable is tested for various disturbance scenarios.Several performance measures are used to evaluate delay propagation in the timetables, includingdeviations from the initial timetable, total delays, total number of delayed trains atdestinations, number of punctual trains with 5 minutes arrival delay tolerance at destinationsand number of violations in trains’ overtaking orders. Results show that the modifiedtimetables outperform the initial ones for small disturbances.

    Keywords
    Robustness, Railway Timetable, Ex-post measures, Delay propagation, Performance measures
    National Category
    Transport Systems and Logistics
    Identifiers
    urn:nbn:se:liu:diva-117042 (URN)
    Conference
    6th International Conference on Railway Operations Modelling and Analysis - RailTokyo2015, Tokyo, March 23-26, 2015
    Available from: 2015-04-13 Created: 2015-04-13 Last updated: 2018-08-23
  • 2.
    Khoshniyat, Fahimeh
    et al.
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Peterson, Anders
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Improving Train Service Reliability by Applying an Effective Timetable Robsutness Strategy2017In: Journal of Intelligent Transportation Systems / Taylor & Francis, ISSN 1547-2450, E-ISSN 1547-2442, Vol. 21, no 6, p. 525-543Article in journal (Refereed)
    Abstract [en]

    To avoid propagation of delays in dense railway timetables, it is important to ensure robustness. One strategy to improve robustness is to provide adequate amount of buffer times between trains. This study concerns how “scheduled minimum headways” should be determined in order to improve robustness in timetables. Scheduled minimum headways include technical minimum headway plus some buffer time. We propose a strategy to be implemented in timetables at the final stages of planning and prior to the operations.  The main contributions of this study are 1) to propose a strategy where the size of the scheduled minimum headways is dependent on trains' travel times instead of a fixed-sized time slot and it is called “travel time dependent scheduled minimum headways” or TTDSMH, 2) to evaluate the effects of the new strategy on heterogeneity, speed, and the number of trains in timetables, 3) to show that a simple strategy can improve robustness without imposing major changes in timetables. The strategy is implemented in an Mixed Integer Linear Programming framework for timetabling and tested for some problem instances from Sweden. Results show that TTDSMH can improve robustness. The proposed strategy can be applied in intelligent transportation tools for railway timetabling.

  • 3.
    Khoshniyat, Fahimeh
    et al.
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, The Institute of Technology.
    Peterson, Anders
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, The Institute of Technology.
    Robustness Improvements in a Train Timetable with Travel Time Dependent Minimum Headways2015Conference paper (Other academic)
    Abstract [en]

    In a railway network with dense traffic, trains’ scheduled arrival and departure times arehighly dependent on each other and even a small delay easily propagates to subsequenttrains using the same infrastructure resources. In the current paper a given timetable is comparedto a modified timetable, where the assigned minimum time slot in the traffic for aservice is linearly increasing with the service’s travel time. The underlying assumption isthat trains lose precision as they travel longer and catching a fixed-size time slot is easier atthe beginning of the journey. Real world observations confirm this assumption as well. Theaim of this study is to verify the improvement in the robustness of those timetables that aremodified with respect to the idea of travel time dependent reserved time slots for the arrivaltimes of trains and to compare the results with the initial timetables. Numerical experimentsare conducted on a selected double track segment of the Swedish Southern mainline. Fourtimetable case studies are considered for the experiments: off-peak hours and peak hoursin 2011 and 2014, respectively. Each timetable is tested for various disturbance scenarios.Several performance measures are used to evaluate delay propagation in the timetables, includingdeviations from the initial timetable, total delays, total number of delayed trains atdestinations, number of punctual trains with 5 minutes arrival delay tolerance at destinationsand number of violations in trains’ overtaking orders. Results show that the modifiedtimetables outperform the initial ones for small disturbances.

  • 4.
    Khoshniyat, Fahimeh
    et al.
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Törnquist Krasemann, Johanna
    Department of Computer Science and Engineering, Blekinge Institute of Technology, Sweden.
    An Optimization Approach for On-Demand Railway Slot Allocation2017Conference paper (Other academic)
    Abstract [en]

    This paper addresses a significant challenge experienced by infrastructure managers concerninghandling and scheduling slot requests for additional trains, or urgent track maintenance,after the master timetable has been finalized. In congested railway networks, wherepassenger trains share the tracks with freight trains and where the freight train operatorscannot fully predict the actual need for access to train slots long in advance, there is aneed for a flexible and effective timetabling revision process. Since the re-scheduling oftraffic and maintenance is a demanding task, the benefits of using computational schedulingsupport is evident. From the perspective of an infrastructure manager, we propose andexperimentally evaluate an optimization-based approach for assessment and scheduling ofadditional slot requests. When inserting several trains, the relations between time and routeoverlap as well as direction of trains, and the required computation time are investigated.The optimization-based approach relies on a Mixed Integer Linear Programming formulation.In this model, the explicit capacity restrictions of line segments and station tracks,including track and platform length, are considered. This model also permits bidirectionaltraffic on all lines where relevant. The experimental results show that optimal solutions canbe retrieved quickly in many scenarios, while for certain scenarios the proposed approachis too time-consuming. The required computation time is very dependent on the propertiesof the inserted train and maintenance slots, respectively.

  • 5.
    Khoshniyat, Fahimeh
    et al.
    Linköping University, Department of Science and Technology, Communications and Transport Systems. Linköping University, Faculty of Science & Engineering.
    Törnquist Krasemann, Johanna
    Department of Computer Science and Engineering, Blekinge Institute of Technology, Sweden.
    Analysis of Strengths and Weaknesses of a MILPModel for Revising Railway Traffic Timetables2017In: 17th Workshop on Algorithmic Approaches for Transportation Modelling, Optimization, and Systems (ATMOS 2017) / [ed] Gianlorenzo D'Angelo and Twan Dollevoet, Dagstuhl, Germany, 2017, Vol. 59, p. 10:1-10:17Conference paper (Refereed)
    Abstract [en]

    A railway timetable is typically planned one year in advance, but may be revised several times prior to the time of operation in order to accommodate on-demand slot requests for inserting additional trains and network maintenance. Revising timetables is a computationally demanding task, given the many dependencies and details to consider. In this paper, we focus on the potential of using optimization-based scheduling approach for revising train timetables during short term planning, from one week to few hours before the actual operation. The approach relies on a MILP (Mixed Integer Linear Program) model which is solved by using the commercial solver Gurobi. In a previous experimental study, the MILP approach was used to revise a significant part of the annual timetable for a sub-network in Southern Sweden to insert additional trains and allocate time slots for urgent maintenance. The results showed that the proposed MILP approach in many cases generates feasible, good solutions rather fast. However, proving optimality was in several cases time-consuming, especially for larger problems. Thus, there is a need to investigate and develop strategies to improve the computational performance. In this paper, we present results from a study, where a number of valid inequalities has been selected and applied to the MILP model with the aim to reduce the computation time. The experimental evaluation of the selected valid inequalities showed that although they can provide a slight improvement with respect to computation time, they are also weakening the LP relaxation of the model.

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